In the future, as wireless data transfer becomes increasingly common and the number of users increases, it is crucially important to increase the capacity of a system by enhancing the performance of the system. One solution to this is to use antenna arrays in transmission and reception, which enables directional beams and antenna diversity to be used. Such beams enable a radio signal to be allocated to a base transceiver station cell area in a controlled manner, which enables a fixed or a dynamically modifiable radio coverage area to be achieved. Typically, beams are formed by means of two or more mutually correlated antenna elements by phasing and possibly by weighting signals received by the antenna elements and signals to be transmitted by such elements. Furthermore, a beam specific signal which, as far as a channel code is concerned, may differ from signals directed at other beams, can be directed at each beam. The stability of a beam structure depends on the stability of the weighting and phasing of a signal directed at each antenna element with respect to the weighting and phasing of signals directed at other antenna elements. Instability is caused e.g. by non-uniform temperature dependence of antenna elements and cabling thereof, in which case a change in temperature causes changes in the phasing and weighting of the antenna elements and, consequently, a change in the beam structure as well. Attempts are made to compensate for the changes caused by instability by calibrating the antenna elements, wherein signals directed at the antenna elements are phased and weighted such that a signal transmitted by the antenna elements is as desired. In some circumstances, however, a calibration procedure may fail e.g. due to defects in the electronic components used in calibration. The situation is particularly problematic when incoming direction estimation is carried out on the basis of insufficiently calibrated beam signals. In such a case, inter-beam handovers and inter-cell handovers and the related signalling may increase significantly, thus decreasing the capacity of the radio system. In the worst case, the entire area of the cell of a base transceiver station becomes inoperative when all connections break down.